Modern scanning techniques permit the observation and examination of structures within the human body. Image analysis methods aid in visually supporting the observations and examinations as well as carrying out measurements.
In order to recognize, measure and evaluate the change in an organic or artificial structure in the human body, it is possible, on the one hand, to separately examine and evaluate the scans at different examination times in order to subsequently compare the assessments, whereby the changes can be recognized. On the other hand, the scanned structures can be brought into a common reference system to subsequently visualize and measure the motion of these structures that has taken place between the examination times and to derive values from the measured data which permit an evaluation of the changes.
In the case of the former approach, size parameters such as volume, length, diameter, circumference, surface etc. are in the prior art often separately measured for the examination times and subsequently compared. In the case of the latter approach, however, often motion fields are calculated from which subsequently global motion parameters can be derived, such as the average displacement or the rotation in various rotational directions which matches best the structure for the different points in time.
Alternatively also local parameters are derived, such as the site of the most considerable deformation and the assigned local motion values.
Specifically in the case of the examination of the change in aorta stent grafts, which are used for the treatment of aorta aneurysms, a method was proposed (e.g. in EP 1 914 683 A2) for calculating the motion relative to a reference structure which does not deform or deforms only slightly by means of the technique of point-set registration or greyscale-picture registration. What is meant by registration is the calculation and performance of the geometric transformation (e.g. displacements and rotations, i.e. rigid transformations or artificially generated deformations according to specific model requirements, i.e. non-rigid transformations) according to which the registered objects match each other to an extent as good as possible. In this method, the stent superimposed for two points in time is illustrated.    1. after only the reference object, e.g. the spinal canal, was registered and the stent was transformed according to the transformation of the reference object, or    2. after a further registration of the stent for both points in time on the basis of the respective position obtained after step 1, wherein the registration may be both rigid and non-rigid.
It is thus possible to quantify both the global motion of the stent graft relative to the reference object and its deformation. The latter is assessed, for example, by the average or the root mean squares of the individual displacement vectors, calculated for all generated stent surface points—or for a selection of these stent surface points. A disadvantage of this analysis is that it is not explicitly determined which deformations occur at which sites of the stent graft. However, this information may be essential, i.a., with respect to the predictive power of the measured values as to whether any complication is looming. Furthermore, the visualization of the superimposed stents is often complicated since the vision on some parts may be obscured by other parts.
It is therefore an object of the present invention to overcome the disadvantages of the prior art and to provide an improved and in particular a more accurate method for visualizing and/or quantifying structures and their changes.
The object of the present invention is solved by the independent claims and the aspects discussed hereinafter. The dependent claims describe further preferred embodiments and modifications of the invention.
Preferred Definitions And Agreements
Surface representation: What is meant by a surface representation is in particular the representation of a surface (initial surface) by means of geometric elements which can be assigned to the surface such that the surface can be re-calculated therefrom at least approximatively—this is described, i.a., in Hoppe et al., “Surface reconstruction from unorganized points” (92). Such geometric elements are, e.g., points which are distributed on the surface and the set of these points thus constitutes the surface representation. The term surface representation preferably is meant to also include the complete initial surface itself as well as substantially the greyscale region of a volumetric picture data set, including the grey value assigned to each voxel of the region, that is completely enclosed by a surface. The definition analogously applies to contours instead of surfaces, specifically in the case of two-dimensional image data sets.
Decomposition of a surface representation: What we understand by decomposition of a surface representation is the fragmentation of the surface which can be reconstructed from the representation into surface components so that essentially every surface area is assigned to exactly one component. If the initial surface is closed, it is possible that, in the event that non-closed components are formed by the above-defined decomposition, optionally additionally a decomposition of the region which was enclosed by the initial surface can be made by means of which substantially such components are supplemented to form closed surfaces. If the initial surface is not closed, it can be firstly closed by adding corresponding surface parts.
The calculation of the decomposition is referred to as automatic if it can be made without any user interaction; if user input is required at one or more points, while the calculation otherwise is made automatically, we refer to the calculation of the decomposition as semi-automatic.
Main axis and main axis portion: If a component of a surface representation is substantially axially symmetric for at least one direction, this surface representation has at least one main axis. A cylinder, for example, has an axis of symmetry, which is in its center—we refer to it as main axis. In the methods described herein, structures are decomposed into components. Preferably, tube-like structures are decomposed into a discrete number of substantially cylindrical or annular components (in the case of a discrete decomposition into annular components, it is possible to define substantially cylindrical components which have the annular components as end parts). Each cylindrical component has a main axis. What is referred to as main axis portion is the part of the main axis that has an extension that substantially corresponds to the length of the cylinder.
Medial line: We define the medial line of an exactly tubular object or its surface representation preferably as being the line through the centers of the circles that are formed in intersections of the object with (perpendicular) planes. In approximatively tubular objects, we refer to each medial line that is obtained by an approximation with an exactly tubular object of the examined object as a medial line. We define the medial line of objects composed of approximatively tubular partial objects as being the composition of the medial lines of each of the individual partial objects, wherein they are joined at the branching points (e.g. Y points) by connection lines, preferably a straight-line section or a smoothly interpolated curve part. We use the term center line synonymously with the term medial line. When the main axis portions have been (approximatively) defined for substantially all components of the object, we understand a medial line of this object also to be the composition of the main axis portions of its components, wherein the ends of the main axis portions are preferably connected by a straight-line section or a smoothly interpolated curve part. Prior to their connection, the two main axis portions to be connected can be extended along the straight lines through the respective main axis portion up to the point where they intersect or up to the points of the shortest distance between the straight lines. Thereafter, they will be connected from these points preferably again by a straight-line section or a smoothly interpolated curve part.
Medial axis or skeleton: A set or a region generally has an edge consisting of edge points. When the region corresponds to a two-dimensional image region, the edge of the region is the contour of the region. Every point in the region has at least one edge point that is closest to it. However, there are also points that do not have an unambiguous closest edge point (the central point in a square, for example, has the same distance to all central points of the sides and this distance is smaller than the distance to all other points of the sides). The set of the points which do not have an unambiguous closest edge point is referred to as medial axis or skeleton. This is described, i.a., in H. Blum: “A transformation for extracting new descriptors of shape” (1967). The medial axis is often a composition of one-dimensional lines which are not necessarily connected. In the aforementioned example of a square, these lines are the diagonals. This definition, of course, applies also to three-dimensional sets or regions. In this case, the medial axis may also be an area. We refer to the medial axis of the area that we obtain as medial axis of the image region as reduced medial axis of a three-dimensional image region. In this connection, the distances between two points of the area are calculated as the lengths of the shortest lines that connect the points within the area.
Confiner: We define a confiner for a given two- or three-dimensional greyscale image data set as a connected region of the image in which all of the greyscales of the associated voxels/pixels are greater than or equal to a predetermined greyscale level and wherein all voxels/pixels adjoining the connected region have a grey value that is strictly smaller than the predetermined level. The fact that the region is connected means that it cannot consist of partial regions which are separated from each other by pixels having a grey value that is strictly smaller than the predetermined level.
Motion vectors: When a point of an object moves from position 1 to position 2, the motion vector of the point is essentially defined as the difference of the local vectors of position 1 and position 2.
“Above” a plane: When a plane is spanned by the vectors a and b, we refer to positions which, in the direction into which the middle finger shows according to the right-hand rule of the Cartesian product (wherein a corresponds to the thumb and b to the index finger), are above the plane as “above” this plane.
Stent graft and stent: A stent graft is in particular a tubular, synthetic-fibre-coated metal grid which may be branched and can be inserted into blood vessels, preferably into the aorta. In particular, it is inserted in case of aneurysms. The metal grid alone, without the synthetic-fibre coating, is referred to as stent. In a CT scan, only the metal grid, i.e. the stent, is visible.